2.4.7 Large-scale singularities

Large-scale singularities are extreme, sometimes irreversible, changes in the Earth system such as abrupt cessation of the Atlantic Meridional Overturning Circulation (MOC) or melting of ice sheets in Greenland or West Antarctica (see Meehl et al., 2007; Randall et al., 2007; also Chapter 19, Section 19.3.5). With few exceptions, such events are not taken into account in socio-economic assessments of climate change. Shutdown of the MOC is simulated in Earth system models of intermediate complexity subject to large, rapid forcing (Meehl et al., 2007; also Chapter 19, Section 19.3.5.3). Artificial ‘hosing’ experiments, assuming the injection of large amounts of freshwater into the oceans at high latitudes, also have been conducted using AOGCMs (e.g., Vellinga and Wood, 2002; Wood et al., 2003) to induce an MOC shutdown. Substantial reduction of greenhouse warming occurs in the Northern Hemisphere, with a net cooling occurring mostly in the North Atlantic region (Wood et al., 2003). Such scenarios have subsequently been applied in impact studies (Higgins and Vellinga, 2004; Higgins and Schneider, 2005; also see Chapter 19, Section 19.4.2.5)

Complete deglaciation of Greenland and the West Antarctica Ice Sheet (WAIS) would raise sea level by 7 m and about 5 m, respectively (Meehl et al., 2007; also Chapter 19, Section 19.3.5.2). One recent study assumed an extreme rate of sea level rise, 5 m by 2100 (Nicholls et al., 2005), to test the limits of adaptation and decision-making (Dawson et al., 2005; Tol et al., 2006). A second study employed a scenario of rapid sea level rise of 2.2 m by 2100 by adding an ice sheet contribution to the highest TAR projection for the period, with the increase continuing unabated after 2100 (Arnell et al., 2005). Both studies describe the potential impacts of such a scenario in Europe, based on expert assessments.